My wall mounted electric heater is rated at 208/240V, single phase. There are two wires provided for connection to the power source, red and black. I know I must use #12 wiring and a 20A circuit breaker. How do I wire the heater to the panel and how many wires do I extend to the panel. Does red go to the circuit breaker? Or black?

You may use a #12-2 with ground cable, with black, white, and bare or green wires. Once done, color code the white at both ends to indicate it is live. Connect the black and white to a two pole 20 amp circuit breaker at the panel, and connect the black and white of the cable to the black and red at the heater. And of course, connect the ground to the green screw in the heater, and the ground bar in the panel.

The heater can not be any larger than 3840 watts for a 20 amp 240 volt circuit.